In the field of communication systems and due to governmental requirements (for example ETSI, FCC), the RF spectral bandwidth of transmitters has generally to be limited. Many ways can be provided for that such as in particular lower output power, lower data rates, baseband data encoding, modulation techniques and controlling data transitions. The use of amplitude shift keying (ASK) modulation is inherently wide bandwidth, and it is often necessary to reduce said bandwidth. A solution for that can consist on limiting edge transition data rate in order to reduce higher order harmonics, but nothing is described for that in the prior art. With high efficiency transmitters, which use class-C amplifier, it is more difficult to achieve this given that said transmitters are not linear.
It can be proposed also for example to insert a cap between the gate and drain terminals of an NMOS power amplifier driver. A drawback of such an arrangement is to decrease the efficiency somewhat and to be not flexible for controlling the shape of the edge ramp at power amplifier output. This is hard to optimize the edge rate for all possible data rates, which are used by multipurpose transmitters. Based-band waveform shaping is difficult to use in class-C type transmitters due to non-linearities in the output.
We can cite the U.S. Pat. No. 7,560,989 B2, which describes a power amplifier circuit with controlled output power. Said power amplifier circuit is shown in part in FIGS. 1 and 2. The power amplifier circuit includes first of all, a power amplifier core 1 to be connected to an antenna assembly 30 shown in dotted lines, at the output terminal PA—OUT. The power amplifier core 1 is composed of parallel amplifier cells A, B, C, D, E1, E4, connected between an earth terminal and the output terminal PA—OUT. Said amplifier cells are preferably cascode amplifier cells, which are each composed with two series connected NMOS transistors N1″ and N3″. A power controller 12 of the power amplifier circuit selects a combination of amplifier cells to activate. The desired power level is determined by a codeword, which defines the combination of amplifier cells to be activated and adjustments of the current and voltage references if needed. In order to activate an amplifier cell, the gate of a second transistor N3″ is controlled by a high level voltage of a supply voltage source, for example to the battery voltage not shown. Each combination of activated amplifier cells defines a predetermined attenuation level of the power amplifier output signal, so that it may be attenuated in a stepwise manner according to the selected combination.
The power amplifier circuit still includes a replica cell 8, similar to any of amplifier cells of the amplifier core 1, a current generator 9 and a voltage generator 11. Said current generator 9 and voltage generator 11 are connected to the power controller 12. The current generator 9 provides a reference current IRef, and the voltage generator 11 provides a reference voltage. The reference current and the reference voltage are generated internally using a band-gap reference. However, another method would be to use an external resistor for example to obtain a more precise and/or flexible solution.
The reference current IRef can be mirrored in the replica 8 by a current mirror 13. The replica cell is advantageously a cascode amplifier cell comprising two transistors NMOS N1 and N3 connected in a cascode arrangement, where transistor N1 is present to limit the voltage on N3. The current K·IRef flowing through the replica cell corresponds to the reference current IRef mirrored in the current mirror 13 with an adequate coefficient K. By monitoring and controlling the current through replica cell 8 a proportional current is to be expected to flow through amplifier core 1, namely the cascode amplifier cells. The related currents IA, IB, IC, ID, IE1, IE4 can be flowed through the cascode amplifier cells, if they are all activated. This is accomplished by using a feedback circuit around the replica. This feedback loop comprises the voltage regulator 10 to fix the top voltage of replica cell 8 to a reference voltage provided by a voltage generator 11.
The regulator 10, namely in this example an operational trans-conductance amplifier (OTA), is used to fix the voltage at the top of the replica cell 8 to a selected reference voltage VRef provided by the voltage generator 11. The power controller 12 is used to adjust both the selected voltage reference and the selected current reference of the current generator. The output of the regulator voltage known as the regulator output voltage is then distributed to provide the supply voltage for the inverter preamplifier 3 as shown in FIG. 2. Said inverter 3 receives in input a signal RF_in and the output of the inverter is connected to all the gates of the first NMOS transistors N1″ of the cascode amplifier cells A, B, C, D, E1, E4. Said signal RF_in is a signal with a carrier frequency for the RF signals to be transmitted, but the ASK modulation of said signal RF_in is obtained by turning on and off the regulator, which supplies said inverter 3. Said ASK modulation is generally an ON-OFF keying. As already mentioned and will be seen later on again, the main power control is accomplished by switching in parallel a combination of selected amplifier cells to define a rough attenuation. In addition, to more finely adjust the attenuation of the power amplifier output signal, the current generator advantageously delivers several different current levels, e.g. four current levels, under request of the power controller allowing such fine attenuation step. Thus the power amplifier output signal may be attenuated in a fine stepwise manner, by controlling the current flowing through the replica device so that it causes the regulator voltage to adjust its output voltage until it equalizes this current. In summary this current level adjusts the supply voltage reference for the inverter.
With such a power amplifier circuit shown in part in FIGS. 1 and 2, the output power can be adapted as a function of the selected cascode amplifier cells, which are controlled by the power controller 12. The reference current and the reference voltage can be also adapted by the power controller in order to optimize performances such as transmission range for the ASK RF signals transmitted by the antenna. However nothing is provided for reducing the bandwidth, if ASK modulation RF signals have to be transmitted with any transition data rate.